1. Field of the Invention
The present invention relates to a switching amplifier and, more particularly, to a tri-state switching amplifier capable of reducing zero-crossing distortion and enhancing efficiency.
2. Description of the Related Art
Conventional switching amplifiers are controlled by a pulse-width-modulation (PWM) signal generated in response to an input signal and an oscillating signal. Transistors of the switching amplifier are turned on or off under the control of the PWM signal, for generating a wave train with discrete pulses of the same amplitude. The discrete pulses are transformed to a continuous waveform through a low-pass filter circuit for accomplishing the magnification of the input signal. The conventional switching amplifier may be classified into a bi-state switching amplifier and a tri-state switching amplifier.
For the bi-state switching amplifier, a positive-polarity state and a negative-polarity state are respectively operated with the same duty of 50% when the input signal is near zero-crossing in amplitude. Based on the balance between the positive-polarity state and the negative-polarity state, an output signal with zero amplitude is obtained, unfortunately, with a drop in efficiency due to unnecessary energy waste.
FIG. 1(a) is a circuit block diagram showing a conventional tri-state switching amplifier 10. FIG. 1(b) is a waveform timing chart showing an operation of the conventional tri-state switching amplifier 10. The tri-state switching amplifier 10 enhances the power of an input signal AU, such as an audio signal, and then supplies it to a load Ld, such as a speaker device. An oscillating circuit 11 applies an oscillating signal T to a tri-state PWM signal generator 12. In response to the input signal AU and the oscillating signal T, the tri-state PWM signal generator 12 generates a tri-state PWM signal 3PS having three states: HIGH, ZERO, and LOW, as shown in FIG. 1(b). When the tri-state PWM signal 3PS is at HIGH, a switch drive logic circuit 13 turns on both of switches S1 and S4, resulting in a potential difference VAB with a positive polarity across the output terminals A and B. When the tri-state PWM signal 3PS is at ZERO, the switch drive logic circuit 13 turns on both of switches S1 and S3 or both of switches S2 and S4, resulting in a potential difference VAB of zero across the output terminals A and B. When the tri-state PWM signal 3PS is at LOW, the switch drive logic circuit 13 turns on both of switches S2 and S3, resulting in a potential difference VAB with a negative polarity across the output terminals A and B. Through a low-pass filter circuit 14, the potential difference VAB is transformed to a continuous waveform output signal for being supplied to the load Ld.
The tri-state switching amplifier 10 is operated in the positive (or negative) polarity state only in a situation that a voltage with the positive (or negative) polarity is required to be supplied from the output terminals A and B. Consequently, the tri-state switching amplifier 10 is operated in the positive (or negative) polarity state with a tiny duty when the input signal AU is near zero-crossing. Although the efficiency is enhanced, the tri-state switching amplifier 10 is subjected to another drawback that the output potential difference VAB is distorted to become a triangular pulse instead of an ideal rectangular pulse when the duty of the positive (or negative) polarity state is reduced until the output voltage VAB fails to completely rise to the full amplitude during a switching period. Furthermore, the rising time and the falling time of the potential difference VAB depend on the characteristics of the switches S1 to S4 and therefore change along with the operating temperature and processing parameters. As a result, the potential difference VAB is difficult to be made free of the zero-crossing distortion.